As is known a contact lens must comply with three functional requirements. First is its optical corrective function which is performed by the central area of the lens. Second, it must be adapted to the wearer's eye, at least in the so-called back off zone which is of greater or lesser extent and is usually situated at the periphery of the contact lens around the central area. The back off zone must define with the wearer's eye a wedge-shaped annular space adapted to facilitate the introduction of a new lacrymal film or tear layer under the contact lens each time the eyelid blinks thereby guaranteeing the sought-after adaptability to the wearer's eye. The functional requirement is wearer comfort particularly when the eyelid closes over the contact lens. It is the configuration of the peripheral margin of the contact lens which ensures wearer comfort too.
At the present the first and third above-mentioned functional requirements may be complied with in a generally satisfactory manner. As for the second functional requirement, wearer adaptability, such is not the case.
As regards so-called hard contact lens, the formation of a peripheral back off zone on the concave side of the contact lens does not create any special problems, as the desired configuration is obtained by grinding.
This is also the case with so-called soft contact lenses which are made of hydrophilic material as they can be ground in a dry state much in the same way as hard contact lenses. Such a substance commonly known under the designation Hema corresponding to hydroxyethyl-methacrylate is a flexible hydrophilic material which may be ground in a dry state.
Contrariwise with soft contact lenses of flexible hydrophobic material, such as silicon rubber, machining is not possible without contrivances for temporary rigidification by contrifugation or cooling, which is hard to perform on an industrial scale. Accordingly, contact lenses of hydrophobic material are cast in molds between two dies, one concave and the other convex.
French patent No. 70 37579 in the name of the assignees of the present application proposed apparatus for direct casting such a contact lens of flexible material into its ultimate desired configuration, whereby any touch ups by machining, which are always difficult to perform for the reasons set out above, are advantageously avoided.
In practice, with a casting apparatus according to the foregoing patent, an annular gasket or sealing member is employed which forms a junction between the dies of the mold at the periphery thereof, and cooperates with the dies to define a molding cavity corresponding to the desired contact lens.
Such a gasket or sealing member is therefore concerned with the concave as well as the convex face. The gasket or sealing member has a glove finger-like configuration extending between peripheries of substantially equal diameters, on the respective mold dies. The gasket or sealing member tapers to near zero thickness to merge tengentially with the dies at their peripheries.
Such an arrangement continues to provide satisfactory results. Nonetheless it has its drawbacks as follows.
First of all, because of its acute profile necessary for its tangential connection with the mold dies, the annular gasket or sealing member is not always easy to fabricate, and its tangential connection with the mold dies does not always provide the desired uniformity.
Moreover, and more importantly, such a sealing member or gasket with a glove finger section does not lead itself to the formation of the back off zone, which for reasons set forth above, is preferably situated at the periphery of the concave face.
As the back off zone is not obtainable with such a gasket or sealing member, it might have been contemplated to form it on the convex die of a complementary shape. Yet, at least for the time being, especially in the optically operative area, there are great practical difficulties if not the impossibility to grind an polish a convex mold die having a hollow circular ridge which forms the intersection of the central area of the contact lens with the peripheral back off zone thereof.